This invention relates to a process for producing a cellulose triacetate film by the solution casting process, and more particularly, relates to a process for producing a cellulose triacetate film increasing the film strength, particularly tear strength and folding endurance.
Cellulose triacetate (TAC) films are widely utilized as the film base for photographic films, and, in general, are produced in solution casting process. In the solution casting process, cellulose triacetate and plasticizer is dissolved in a solvent mixture of which the principal solvent is methylene chloride and some poor solvents of cellulose triacetate (hereinafter called poor solvent(s)) to obtain a dope, and the dope is fed uniformly from a die continuously on a rotating endless support. The solvents are evaporated on the support, and after the dope is solidified, it is stripped from the support. Then, it is dried to obtain the film.
Recently, a method capable of increasing the film producing speed was developed by adding a poor solvent, such as methanol and cyclohexane, ethanol and n-butanol or abundant methanol or ethanol in addition to methylene chloride into the cellulose acetate dope to accelerate the solidification speed (U.S. Pat. Nos. 2,607,704, 2,739,069, Japanese Patent KOKOKU No. 45-9074, Japanese Patent KOKAI No. 54-48862). However, this method is insufficient for shortening the time from the feeding to the stripping and the time after the stripping.
Another method for accelerating the film producing speed of TAC film is that the dope is fed on the endless support of which the surface temperature is kept lower than 10.degree. C. to accelerate the solidification and is stripped off at an early stage (Japanese Patent KOKAI Nos. 62-37113, 62-64514 and 62-115035). According to this method, since the film stripped from the support can be dried from both sides efficiently, it is possible that the time necessary for producing the film is sharply shortened. However, when the film is dried under the conventional conditions, the obtained film is undesirable in its physical strength, particularly folding endurance.
Incidentally, a method for improving the film strength is known (Japanese Patent KOKOKU No. 61-39890). In this method, a dope containing a poor solvent in addition to methylene chloride is fed on the surface of a band support, and the dope layer is heated up to 30.degree.-42.degree. C. within 3 minutes after fed and kept at this temperature at least for 15 seconds to increase the film strength, particularly tear strength. However, the folding endurance of the film obtained in this method is still insufficient. Moreover, the time from feeding the dope on the band surface to stripping off is long, and therefore, a long time is necessary for the production of the film.
Another method for improving the film strength is that the film is dried with keeping the shrinkage percentage to 3-20% (described in Japanese Patent KOKOKU No. 49-5614 as the prior art), but the film producing speed cannot be increased sufficiently by this method.
Still another method is that the shrinkage percentage is increased up to more than 20% by adding a poor solvent abundantly or by drying with keeping the stripped film at a low temperature (Japanese Patent KOKOKU No. 49-4554). However, this method has disadvantages of insufficient film producing speed and the occurrence of wrinkle.
As mentioned above, only one of the physical strength and the producing speed is improved by the aforementioned methods, and a method capable of satisfying both of the physical strength and the producing speed has not developed yet.